The combining of imaging modalities to offer increased functionality has produced a number of useful imaging systems, particularly in medical diagnostic and small animal imaging. For example, Gamma ray PET detector systems are frequently sold with x-ray computed tomography (CT) detector systems (although the PET and CT detector systems are physically separate and therefore do not share detectors or a common imaging space). A notable attempt at offering an integrated imaging system (in which detectors and the imaging space of the system are shared) was a SPECT-PET (nuclear medicine and PET) imaging system which reduced costs by sharing detectors and the imaging space (the volume in which the object is imaged). Although these SPECT-PET imaging systems were not well received commercially due to performance compromises nonetheless they offered interesting functionality since SPECT and PET images could be acquired separately or simultaneously in a shared imaging space (thereby avoiding registration error between separately acquired SPECT and PET images and reducing the total scan time). In addition, simultaneous CT-SPECT systems have been proposed (typically using CZT or CdTe) although issues arise due to generally differing collimation and flux rate requirements.